Allow curves to be manipulated in Python

[NickDraper]

From QTIPLOT handbook:

All the curves in a plot layer can be customized in terms of color, line width and line style. Here's a short script showing the corresponding functions at work:

t = newTable("test", 30, 4)
for i in range(1, t.numRows()+1):
    t.setCell(1, i, i)
    t.setCell(2, i, i)
    t.setCell(3, i, i+2)
    t.setCell(4, i, i+4)

l = plot(t, (2,3,4), Layer.Line).activeLayer() # plot columns 2, 3 and 4
for i in range(0, l.numCurves()):
    l.setCurveLineColor(i, 1 + i) #curve color is defined as an integer value
    l.setCurveLineWidth(i, 0.5 + 2*i)

l.setCurveLineStyle(1, QtCore.Qt.DotLine)
l.setCurveLineStyle(2, QtCore.Qt.DashLine)

[NickDraper]

This issue was originally trac ticket 2655